1. Field of the Invention
The present invention relates to an electric power steering apparatus, and particularly to the electric power steering apparatus with less vibration of a steering wheel and less noises, and with satisfactory steering feeling. Further, the present invention relates to the electric power steering apparatus having satisfactory steering feeling in which vibration and noise (hereinafter, called also as “steering holding noise”) on a steering wheel in case of steering holding state can be reduced and a steering performance in a normal steering state (at the time of normal steering) can be secured.
2. Prior Art
Electric power steering apparatuses, which apply an assist torque to a steering apparatus of a vehicle by means of a rotation torque of a motor, apply the assist torque to a steering shaft or a rack shaft in such a manner a transmission mechanism such as gears or a belt applies a driving power of the motor via reduction gears. A simple constitution of such an electric power steering apparatus is explained with reference to FIG. 1.
A shaft 102 of a steering handle 101 is connected with a tie rod 106 of a steering wheel via reduction gears 103, universal joints 104a and 104b, and a pinion rack mechanism 105. The shaft 102 is provided with a torque sensor 107 that detects a steering torque of the steering handle 101, and a motor 108 that assists a steering torque of the steering handle 101 is connected to the shaft 102 via the reduction gears 103.
A control unit of the electric power steering apparatus having such a constitution is explained with reference to FIG. 2. A torque value T detected by the torque sensor 107 and a vehicle speed V detected by a vehicles speed sensor (not shown) are inputted to an assist map 190, and a steering assist command value is calculated. Further, a compensation value to be calculated by a compensation value calculating section 194 such as compensation values of convergence calculated by a convergence calculating section 191, inertia calculated by an inertia calculating section 192, self-aligning torque (SAT) and the like are added to the steering assist command value by adding sections 195, 196 and 197 so that a torque command value Tref is determined. A current command value calculating section 200 determines a current command value Iref based on the torque command value Tref. In a brushless motor, an angle of a rotor as well as the torque command value is inputted to the current command value calculating section 200, so that the current command value Iref is determined. A process portion up to the step of determining the current command value Iref based on the torque value T, the vehicle speed V and the compensation (portion surrounded by a broken line B) is called as a current command value determining section B for convenience.
On the other hand, a motor current Im to be supplied to the motor 108 is detected by a current detector 202, and it is inputted together with the current command value Iref to a subtracting section 204. The subtracting section 204 calculates an error ΔI=Iref−Im between them.
The error ΔI is inputted to a proportional-plus-integral (PI) control section as a current controller A surrounded by a broken line A. In this example, the error ΔI is inputted to a proportional section 208 as a proportional gain Kp and an integral section 206 as an integral gain Ki. An output from the proportional section 208 and an output from the integral section 206 are added by an adding section 210, and a voltage command value Vref is outputted.
A PWM (Pulse Width Modulation) control section 212 inputs the voltage command-value Vref therein, and outputs a PWM signal to an inverter circuit 214, so that the PWM signal based on the voltage command value Vref is instructed to the inverter circuit 214. The inverter circuit 214 supplies a motor current Im to the motor 108 based on the PWM signal.
The above explanation refers to the example where the proportional-plus-integral control is used in the current controller A in the control of the electric power steering apparatus. A steady state gain Gi relating to the proportional-plus-integral is expressed by the following equation 1.
                                        Gi                          =                                            lim                              s                ->                o                                      ⁢                                                  ⁢                          (                              Kp                +                                  Ki                  /                  s                                            )                                =                      ∞            .                                              [                  Equation          ⁢                                          ⁢          1                ]            
That is to say, even when the error ΔI has a small value, the steady state gain Gi is infinite. For this reason, the voltage command value Vref is outputted as a large value even in case of steering holding state of a steering wheel or slow steering, and the following problems arise.
That is to say, recent electric power steering apparatuses provide high output, namely, large electric current, but A/D converters or the like which are used in control devices composed mainly of CPU have, for example, still 10 bits, and thus their resolution becomes relatively rough at the time of large current. A calculating error such as quantization error caused by the roughness of the resolution is amplified by a feedback gain, which is obtained when the steady state gain becomes infinite at the time when the error ΔI at the steering holding state or slow steering state is small, and it becomes vibration or noise of the steering wheel, thereby bringing discomfort to drivers.
In the control of the electric power steering apparatus, the integral control (I control), the proportional-plus-integral control (PI control) and proportional-plus-integral-plus-derivative control (PID control) as the current control section including an integral are generally used. In the control including the integral, however, since its gain becomes infinite in a steady state, in the case where the electric power steering apparatus is in the steady state, namely, the steering holding state or in the slow steering state, the driver feels vibration or noise via the steering wheel and the driver has discomfort about the steering of the steering wheel.
In Japanese Patent application Laid-open No. 2000-108916 A, in order to solve the above problems, in the feedback control, an adjustment gain is provided before the error ΔI=Iref−Im between the current command value Iref and the actual motor current Im is inputted to the control system including the integral element. When the error ΔI is small, the adjustment gain is reduced, and when the error ΔI is large, the adjustment gain is increased.
In the conventional apparatus, however, the design (threshold value) for determining the adjustment gain which changes according to the error of the electric current feedback is difficult, and the steady state gain of the integral element is infinite. For this reason, even if the adjustment gain is reduced, the total gain becomes large, and thus the problem such that the vibration and the noise generated in case of the steering holding state and slow steering state of the steering wheel is not solved.
Further, discrete time, fixed-point arithmetic and the like which are mounting problems in the case where the CPU is utilized as a control device are not taken into consideration.